The present invention relates to a magnetic arrangement for beam position adjustment in a multiple-beam cathode ray tube, and more particularly, to a magnetic arrangement for adjusting static convergence of the plurality of electron beams in an in-line, tri-beam, color cathode ray tube.
In a tri-beam color cathode ray tube employing an in-line beam configuration, the electron beam sources are aligned to originate beam paths having axes lying essentially in a common plane, with a central beam path oriented in registry with the tube neck axis and with respective outer beam paths symmetrically disposed on opposite sides of the central beam.
For proper picture reproduction, it is desired that the three beams strike coincident regions of the phosphor screen of the cathode ray tube. While the gun structures of the cathode ray tube are designed ideally to effect such convergence of the beams at the screen center in the absence of beam deflection, practical tolerances in the manufacture of the cathode ray tube and associated components dictate the need for associating with the cathode ray tube suitable means for correcting a range of center-of-screen misconvergence errors that may be encountered in actual practice.
For this purpose, there have been proposed various magnetic arrangements for correcting the misconvergence and, one of which is disclosed in U.S. Pat. No. 3,725,831 issued to Barbin on Apr. 3, 1973. According to this reference, a pair of juxtaposed four-pole magnet rings and a pair of juxtaposed six-pole magnet rings are rotatably mounted about axially spaced regions of a tube neck section of the cathode ray tube.
Another arrangement is disclosed in Japanese Utility Model Publications No. 26593/1976 and No. 26594/1976 both issued to Yamada et al. on July 6, 1976. According to these references, a U-shaped member A carrying five permanent magnets M1, M2, M3, M4 and M5, as shown in FIG. 1, is rotatably mounted on the tube neck section. The magnetic flux (dotted line) produced between the magnets M2 and M4 affect on the magnetic flux (solid line) produced between north and south poles of the magnet M3 in such a manner that the outer magnetic flux from the magnet M3 is counterbalanced with the flux between the magnets M2 and M4 while the inner magnetic flux covering axial beam location B is aided with the flux between the magnets M2 and M4. Accordingly, the field at beam location B is directed downward to produce a leftward shift of the electron beam at location B, provided that electron motion is out from the plane of the paper in the view of FIG. 1. According to this arrangement, since the available magnetic flux direct only in one direction, i.e., from top to bottom in FIG. 1, the beam shift at location B can be directed, .+-.90.degree. from the arrow direction shown in FIG. 1 with respect to the rotation of the U-shaped member about the tube neck section. If it is required to shift the beam at location B rightwardly, it is necessary to draw out and turn over the U-shaped member A and then mount again the U-shaped member A to direct the available magnetic flux from bottom to top. Therefore, the adjustment for the beam shift direction required a number of steps, particularly when right and leftwards shift is involved.
The above mentioned arrangement has a disadvantage not only in the steps for adjusting the beam shift direction but also in the manufacturing steps particularly, the steps for mounting the magnetic pieces on the U-shaped member A.